The present invention relates to a ROM (Read-Only Memory) cell structure, and more particularly to a late programming mask ROM integrated circuit (IC) having new memory cell structure, and a process for producing the same.
ROMs have been widely used in digital equipment, such as minicomputers and microcomputers, microprocessor system, and the like, to store fixed programs and/or data for the systems. A ROM is a fixed-type or non-volatile memory device, and it is programmed during fabrication. In general, customers must send their programs and/or data to a ROM manufacturer before devices are manufactured. Then, the ROM manufacturer produces and delivers finished ROM ICs with desired programs and/or data permanently stored therein to the customer. The manufacturing process for ROM integrated circuits is very complicated and lengthy, and requires a large number of steps, with each step taking up much time and complicating material handling problems and inventory factors. Since most ROM devices are identical, except for the information stored or programmed therein, the ROM production process usually has two major phases. In a first phase, all processing steps till the step right before the programming step (which is near the end of entire ROM producing process) are performed. Then, the partially finished ROM devices are stockpiled to await program orders from customers. Upon receipt of an order, a program mask is prepared corresponding to a customer's program and/or data. In phase two, the programming step occurs, and the remaining process steps are then completed in a short period of time. In this manner, ROM ICs can be finished and delivered quickly after a customer places an order. Thus, late programming mask ROM techniques are commonly used by ROM IC manufacturers.
At present, there are many methods which may be used to program ROM devices near the end of the device fabrication process. For example, the programming step can be completed by (1) ion implantation into the channel regions of selected transistor devices through their gate electrodes to alter their threshold voltages, or (2) leaving out selected contact openings in order not to provide electrical contacts to the selected memory cells, or (3) directly changing the metal pattern so as to bypass the selected memory cells. These programming methods, however, have their respective disadvantages.
Method (1) requires very high energy implantation to implant dopants through the gate oxide, typically relatively thick polysilicon gate electrode and through other films or layers which may exist on top of the polysilicon at this state. It is difficult to properly control the threshold voltages of the devices because (i) too high of an implant dose may adversely affect the breakdown voltage of adjacent junctions and (ii) unexpectedly thick gate electrodes may block the implant dose resulting in an insufficient change of the threshold voltage. Thus, Method (1) is difficult to control.
Method (2) requires a relatively large chip layout area because the ROM has to be so designed that each memory cell has its own contact opening, and contacts cannot be shared between cells. Similarly, Method (3) requires extra spaces on the chip because regular metal runs must be interrupted to make a detour and bypass the nonselected contacts. Methods (2) and (3) require that the ROM be unduly large in size.